Biomass is emerging as a possible renewable alternative to petroleum-based resources in light of increasing environmental, economic and political difficulties associated with fossil fuel extraction and use. Accordingly, biomass-derived sugars have been presented as intermediates for the production of renewable fuels (1-3) and chemicals (4-6). However, producing water-soluble carbohydrates from lignocellulosic biomass requires cleaving ether bonds in xylan and glucan chains, while minimizing further degradation of the resulting C5 and C6 sugars (xylose and glucose) to insoluble degradation products. (See, for example, FIG. 1A). Unfortunately, in aqueous solutions containing low acid concentrations (<10 wt %), the high rate of sugar degradation reactions compared to polysaccharide depolymerization necessitates impractical reaction protocols for converting solid biomass, such as short residence times (10 ms to 1 min) at high temperatures (520-670 K) to obtain high yields of glucose (7). Due to the recalcitrance of crystalline cellulose to deconstruction, high yields at lower reaction temperatures can only be obtained using concentrated mineral acid and/or ionic liquids (8, 9). However, recovery of the mineral acid is critical to the economics of the process, and the cost of ionic liquids can be prohibitive (8-10). Similarly, cellulase enzymes operating at temperatures of 320 K can achieve high glucose yields when converting cellulose rendered accessible by thermochemical pretreatment. However, the costs associated with producing these enzymes can be substantial compared to the value of the final product (with estimates of $0.35 to 1.47 per gallon of lignocellulose-derived ethanol (11, 12)).
Decoupling the residence times of the solid carbohydrate polymer from its soluble counterpart by flowing a solvent through a heated packed bed of biomass can minimize sugar degradation when using low acid concentrations (13). These systems are limited by their inability to produce concentrated soluble carbohydrate solutions (e.g., 45 to 55% glucose yields when producing a 2 to 4 wt % sugar solution using 1 wt % H2SO4 in water (13)). In recent work, it has been shown that liquid solutions of gamma-valerolactone (GVL) and water containing dilute concentrations of mineral acids (<0.1 M H2SO4) can dissolve lignocellulosic biomass and be used to produce levulinic acid and furfural (6, 14).